This was one of the most challenging interviews I've been a part of. Although I didn't get the position the whole process was still worth the experience and practice.
The first phone interview with the requiter was quick and easy. I was then given a test project to work on. I was asked to redesign and write html/css for an ecommerce product page's Rating and Reviews section (which is their product). After spending about 15-20 hours on it I turned it in was was one of the ones chosen for an interview. I then had 3 one-on-one half hour interviews. I had to defend my design decisions in great detail. The director was pretty strict and wanted very thought out answers, no fluff. I also had to speak CSS code for an on the spot test. They wanted me to speak on several problem solving stories from past experiences.
Although my communication skills didn't quite win them over, it was a good exercise. I'd advise you not only need the portfolio and skills but have your best salesman attitude and design critique vocabulary ready.